Thank you for taking the time to reply and give me some insight into how you developed the game and your thought process.
I am a pretty novice sudoku player - it is my in-flight game of choice for when I'm on an airplane. But having it on the go with the playdate (and yes, I'm stupid lucky to even have one right now) has proved to be a fun little boon to my playtime with the game.
That said, I might not be the best to provide deeper insights into development from within the sudoku world, but I did have one other feature I was hoping for, if it is in your interest or desire to flesh out - a way to leave potentials marked in the cells (and maybe this is just a novice sudoku player thing to even want). The way I've done this in the past, is with smaller numbers filling out the top of the cells, before eventually (hopefully) eliminating the ones that no longer apply, and filling out the square with the right number.
As far as implementation, I sat down and worked through some ideas, if you are interested. I worked out (in photoshop) that a 6pt font would allow the requisite 9 potentials to be marked inside the cell (3x3) - though I clearly have no idea if this translates to the development/coding side of things. I also thought maybe an A+B combo press could be used to access/exit this sub cell marking area (as long-presses are already used for fast scrolls), wherein the normal controls then are used marking. I was thinking the sub cell text starts top left, accessing top middle after a number is placed and the right arrow key is pressed, proceeding to top right, middle left, etc. Highlighting, in the way you've already done, may be necessary for entering and editing the potentials, with null entries in the list being voided (say you figure out that potential is no longer valid), and subsequent potentials shifted. Automatic ascending sorting may also be useful in the sub cell. And I thought that if the user marks the cell as normal (without going into the sub cell) with their guess, it erases the potentials, but saves them, and reloads them if the user erases their guess (which, sounds like it would be annoying to implement, as someone who doesn't code).
Anyway, that's my latest thought - please completely ignore if you don't see it as viable, valuable, manageable or whatever - my feelings will not be hurt. Again, I'm thankful for your time already devoted to this, and for replying to me, and hope you get to see it in use in your own hands very soon. Thanks :)